In pumping concrete or similar high viscosity mud-like material, considerable power is required to maintain a reasonable flow rate. Various types of impellers, augers and the like have been used, but any mechanism which requires close tolerance fits between parts is at a disadvantage. Concrete and other mud material tends to clog any mechanism it comes in contact with and, if allowed to set, can render the mechanism inoperative. Most apparatus in present use requires constant maintenance and cleaning to keep it operable.